Spring Data JPA Repositories

Detailed answer

Logic: Interface extending JpaRepository<T,ID> gets CRUD + query methods from method names. @Query for JPQL/native; Pageable for pagination. Complexity notes included in code section when present. Pros: Minimal boilerplate; pagination and sorting built-in. Cons: Derived query names explode; N+1 if fetch graphs missing. Core: Interface extending JpaRepository<T,ID> gets CRUD + query methods from method names. @Query for JPQL/native; Pageable for pagination. Real-time example: findByEmailIgnoreCaseAndActiveTrue(String email) generates query at startup. Pros: Minimal boilerplate; pagination and sorting built-in. Cons: Derived query names explode; N+1 if fetch graphs missing. Common mistakes: Long derived method names unreadable; missing @Transactional on service layer writes. Best practices: Use @EntityGraph or JOIN FETCH for associations; DTO projections for reads. Audience level: Fresher.

Full explanation

Interface extending JpaRepository<T,ID> gets CRUD + query methods from method names. @Query for JPQL/native; Pageable for pagination.

Code example

public interface UserRepo extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);

    @Query("select u from User u join fetch u.roles where u.id = :id")
    Optional<User> findWithRoles(@Param("id") Long id);

    Page<User> findByActiveTrue(Pageable pageable);
}

Practice code · java (view only · no execution)

public interface UserRepo extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);

    @Query("select u from User u join fetch u.roles where u.id = :id")
    Optional<User> findWithRoles(@Param("id") Long id);

    Page<User> findByActiveTrue(Pageable pageable);
}